1. Field of the Invention
The present invention relates to an apparatus and a method for detecting a short-circuit (or a "short") in a battery cell.
2. Description of the Related Art
When a battery cell is used in a system, a single battery cell may not produce a sufficient electric power to drive the system. Therefore, normally, a plurality of cells are serially connected together to produce a sufficient electric power. Such a group of serially-connected cells, which is capable of supplying a sufficient electric power to drive the system, is called a "battery pack", and the individual cells are also called "batteries".
A hybrid electric car uses a battery pack which includes as many as 240 battery cells. In a hybrid electric car, detection of a "cell short" (as defined below) is not performed because the detection cannot be performed by means of a conventional cell short determination method. Conventional cell short determination methods used in other systems include the following methods.
Japanese Patent No. 2,754,506 discloses a method in which a short of a single cell is determined by forcibly charging a certain amount of electricity into a plurality of serially connected batteries and then determining whether the voltage of the serially connected batteries can be increased over a predetermined value. Japanese Patent No. 1,253,012 discloses a method in which a cell short is determined by detecting a sound of bubbling from the electrolytic solution which is generated when rapidly discharging the battery.
FIG. 5 illustrates the conventional cell short determination method disclosed in Japanese Patent No. 2,754,506. In order to determine a cell short in a battery, the battery is charged for a predetermined period of time T. Then, the battery voltage is detected after the passage of the period of time T, and the detected battery voltage is compared with a predetermined reference voltage Vb. It is determined that at least one of the battery cells has a short when the battery voltage is not equal to or greater than the predetermined reference voltage Vb.
Where it is not possible to detect a single cell short which has occurred in a battery pack, there are a number of problems as follows.
1. Cells in a battery pack are grouped into blocks by several cells so as to reduce the influence of a difference in a remaining capacitance among the cells caused by a temperature distribution across the battery pack. Estimation of the capacitance of the battery is performed for each block by using a voltage as an index. However, the remaining capacitance may not be estimated accurately in a block including a shorted cell due to a low voltage. Therefore, even if the block including a shorted cell is in a state of being overcharged, the block is charged further. As a result, operational lives of even normal cells in blocks other than the block including a shorted cell are shortened by the overcharge.
2. The voltage of the block including a shorted cell appears to be lower than that of the remaining capacitance. Therefore, the capacitance of the battery pack is determined to be low even if a sufficient capacitance is remaining in the other normal cells. As a result, discharging of the whole battery pack is stopped. Thus, the capacitance of the battery pack is not used efficiently.
Where the above-described conventional techniques are used, there are technical problems as follows:
1. In a system such as a hybrid electric car, where battery cells are charged or discharged according to the driver's operation of the accelerator, it is not possible to charge the cells continuously to determine whether a single cell is shorted. Therefore, it is not possible to determine a cell short by using the method of Japanese Patent No. 2,754,506 discussed above in which the battery cells are forcibly charged continuously.
2. It is not desirable in view of cost to provide a microphone on every single cell to detect a sound as in Japanese Patent No. 1,253,012 discussed above.
3. The voltage of a battery varies greatly according to the remaining capacitance and the charging/discharging current. Thus, a voltage drop resulting from a short of a cell is made inconspicuous in comparison with a substantial change in voltage due to the remaining capacitance and/or the charging/discharging current. Accordingly, it is difficult to detect a short of a cell.
The present invention uses a different method from the conventional methods to detect a short of a cell and solve the problems described above.